The present invention relates to an electronic circuit for measuring small amounts of charge or small electrical currents.
Often the measurement of a physical phenomena involves the creation of an electrical signal which is amplified and measured by an electrical circuit. A number of sensing media face limitations, imposed by the electronic measuring circuits, on their ability to measure physical phenomena. These media include solids, liquids, or gases in which the physical phenomena to be sensed causes the generation of mobile charges which then move under the influence of an electric field, or in which a charge separation is induced by the physical phenomena and/or dimensional changes in the sensing medium. Specific examples of sensing media are gases in the case of ionizing radiation, electrochemical mixtures, electret or capacitive microphones, variable capacitors, inductive pickups or coils, electric field measuring antennas, piezoelectric materials such as PVDF, semiconductors operated at temperatures where thermally-induced current generation is not dominant, a vacuum in the case of electron emissive surfaces or ion mobility instruments, and insulators. The limitations imposed by the measuring circuits include sensitivity, linearity, size, dynamic range, and operating voltages. These limitations arise from thermally-induced and/or bias currents in the components of the measuring circuit and current leakage paths over the surfaces of and/or through the components of the measuring circuit. The measuring circuit described herein aims to minimize these limitations.
One example of a physical phenomena is ionizing radiation. It presents a direct hazard to people; therefore, the measurement of radiation in various environmental settings is important. The type of radiation monitor or detector used to measure the radiation depends upon the type of radiation, e.g., beta, alpha, or X-ray and the environmental setting, e.g., an environmentally isolated laboratory, an open mine, or a waste dump holding potentially hazardous material. Different environments impose different requirements on the manner and sensitivity of the measurement; for example, the laboratory most likely requires a monitoring system with a continuous display and singular or multiple radiation detectors; the mine requires a moderately sensitive portable area detector; and the waste dump a relatively fast and sensitive directional detector.
D. A. Waechter et al. described in an article entitled xe2x80x9cNew Generation Low Power Radiation Survey Instruments,xe2x80x9d a standard portable dosimeter (radiation monitor) system. The portable monitor consists of a Geiger-Muller tube (GM tube) with an event counter which records the number of ionizing events. There is a readout display and an audio alarm. The problem with the GM tube is that its response is not linear with the energy of the radiation so its accuracy varies with radiation photon energy, although it is useful for warning. In this instrument, the need to amplify the radiation signal in the GM tube limits the energy linearity and thus the accuracy of the instrument. An ion chamber made from tissue equivalent plastic and filled with a tissue equivalent gas gives a very accurate reading. However, at low doses and dose rates, the amount of charge generated per unit volume of gas is very small. For accurate measurement of the ionizing radiation, the signal current created by the ionizing radiation needs to be significant when compared to the leakage and/or noise currents in the electronic measuring circuit and the internal leakage currents in the gas sensing medium. It is preferred that the signal current be greater than the sum of all the leakage and noise currents. In general, the internal leakage currents of the sensing media, which are induced by physical phenomena other than the one to be measured, must be on the same order of magnitude or preferably less than the currents created by the physical phenomena to be measured in order for any measuring circuit, including the ones described herein, to obtain a measurement. Sensing media with low internal leakage currents are said to have high internal impedances. Gas is a very high impedance sensing medium and so does not contribute significant internal leakage currents. But, due to surface leakage currents on insulators and other circuit elements and due to other limitations within the implementing circuitry, prior art ion chambers tend to need a large volume of gas, and thus operate at high pressure or be inconveniently large, and need to employ high voltages to be sufficiently sensitive. By significantly reducing surface leakage currents and bias currents, this invention allows accurate ionizing radiation measurement with lower voltages and smaller chambers.
A second group of sensors that can benefit from the invention disclosed herein are charge inducing sensors such as capacitive sensors, where a charge is induced if the voltage difference is kept constant, or where a voltage is induced if the charge stays constant. Capacitive sensors can be used in many applications such as, microphones, pressure measurements, and accelerometers to name a few. Books like Capacitive Sensors Design and Application by Larry K. Baxter (ISBN 0-7803-1130-2) give many applications, some examples of which are given on pages 3-5. It is important to note that in capacitive sensors there is a high impedance material, often gas, but sometimes a liquid or a solid, between the capacitor electrodes. In this type of sensor the high impedance material is functioning as a separating medium rather than a sensing medium. The sensing medium is the capacitor plates themselves and the separation between them. There is not meant to be charge conduction through the separating medium. For this reason the term non-mobile charges can be used. The induced charges do move within the conductors connected to the sensing electrodes and the other electrodes, but they do not move through the separating medium. Most prior art capacitive sensors are limited to alternating current (AC) use because of the bias and leakage currents introduced by the prior art measurement electronics or within or over the surfaces of the capacitive sensors themselves. Among the limitations are: (i) response time, since it takes several cycles of the AC signal for a change to be noticed, (ii) power use, because the AC signal require an AC source and constant current draw, and (iii) sensitivity, because the measurement is a small change of a large AC signal.
Another group of charge sensors which can benefit from the invention is electrochemical sensors, where the phenomena to be measured interacts via a reversible or irreversible chemical reaction that causes the formation of charge in electrical dipole layers. The charges need not be mobile because the creation of an electric field by the induced charge distribution will result in a signal which can be measured. In some configurations, electrochemical sensors are low impedance sensors, with significant internal leakage currents inherent in the medium itself. Some water-based electrochemical cells are examples of this. Even in low internal impedance sensors, there may be a sensitivity or linearity benefit from using an electronic circuit which does not load or draw much current from the sensor. In some configurations, the current generated is the electrical signal which most directly relates to the physical phenomena. In other configurations, electrochemical sensors produce a voltage that is measured to represent the physical phenomena. The measurement of work function difference between two surfaces with an insulating medium as a separation material is an example of this. One prior art method for measuring work function difference is a moving plate or vibrating reed electrometer as described in pages 82-83 and pages 407-409 of Nuclear Radiation Detection by William J. Price (Second Edition, Library of Congress Catalog Card Number 63-23463). In this device, mechanically moving or vibrating a capacitor plate induces an alternating current proportional to the voltage difference between the two plates and the time rate of change of the capacitance, I=V*(dC/dt). While this circuit is very sensitive, its usefulness is limited because of the size of the vibrating plates and the power needed to create the vibration or motion. A vibrating reed electrometer is different than the capacitive sensors because in the capacitive sensors, the change in capacitance is related to the phenomena to be measured, while in a vibrating reed electrometer, the capacitance change is periodic and is used to amplify the voltage difference and convert it into a current.
Another set of applications in which the invention may be used are mass spectrometers, flame ionization detectors, vacuum gauges, gas chromatographs, photodetector tubes, and others where charged ions or electrons are moving in a vacuum. These can be termed free electron detectors, although ions of both positive and negative charge may be collected, as well as free electrons. Often high voltage electron multiplication is used to amplify the signal. However, this amplification introduces some statistical noise and the electronics necessary to accomplish the high voltage multiplication are costly. The electrical circuit disclosed in the present invention can reduce noise and costs in these and similar charge measurement applications.
In all measurement or sensing applications where the noise of the electronic measurement circuit limits the lower limit of sensitivity or affects the linearity or dynamic range of the sensor, the electronic circuit presented herein can improve the sensor""s performance. Among types of noise are shot noise, Johnson and other thermal noise sources, bias currents, leakage currents and other charge conduction paths that interfere with accurately measuring or detecting the signal. It is also advantageous to use the circuit proposed here because of reduced cost, size, and/or power.
In U.S. Pat. Nos. 4,769,547, 4,804,847 and 4,970,391, certain circuits are described in relation to ion chamber construction. This patent application extends the teachings of those patents by disclosing circuit configurations and techniques which permit the invention""s use in other applications which can benefit from increased detector sensitivity, linearity, dynamic range, lower cost, etc.
It is an object of the present invention to provide an electronic circuit which improves the performance of sensors which produce small currents, charges, or voltages by improving their lower limits of detection, dynamic range, frequency range, and/or linearity.
It is another object of the present invention to provide an electronic circuit which improves sensor performance by eliminating continuous direct current discharge paths for removal of the charge or current generated by the sensor.
It is another object of the present invention to provide an electronic circuit which improves sensor performance by minimizing inaccuracies caused by bias currents, leakage currents, current drawn from or input to the sensor, and/or other charge conduction paths that interfere with accurately measuring or detecting the signal.
It is another object of the present invention to provide an electronic circuit which improves sensor performance by minimizing inaccuracies caused by shot noise, Johnson noise, thermal noise, and other AC or DC noise sources.
It is another object of the present invention to provide an electronic circuit which improves sensor performance by minimizing inaccuracies caused by the dead time during which the charge measurement circuit is being discharged or restored.
It is another object of the present invention to provide an electronic circuit for measuring current or charge.
It is another object of the present invention to measure a low current or low voltage signal and provide a direct digital output.
It is another object of the present invention to reduce the power required to measure a low current or low voltage signal.
It is another object of the present invention to provide an electronic circuit for use with a variety of sensing media, including but not limited to high impedance sensing media, that produce a signal by either charge or current, production or induction in response to some physical phenomena occurring within the sensing media.
It is another object of the present invention to provide small, lightweight assembly which electronically records the phenomena being sensed.
It is an additional object of the present invention to provide an electronic circuit with a transistor and/or integrated (electronic) circuit design, manufacture, and handling process that optimizes the ability to operate with small currents, small amounts of charge and high impedance sensing mediums.
One embodiment of the present invention provides an electronic circuit for measuring current or charge that can be used with a variety of sensing media (including high impedance sensing media) that produce a signal by either charge or current production or induction in response to physical phenomena occurring within the sensing media. A current or charge sensing or collecting electrode is placed within the sensing medium to create a signal from the produced or induced current or charge that is electrically coupled to a control gate of an amplifying transistor which is either connected to or incorporated into an amplifier. The signal is created by establishing an electric field that moves charge of one polarity toward the sensing electrode. The electric field is established by setting a reference electrode at a different voltage potential (bias) than the sensing electrode. The control gate/sensing electrode, which is initially biased to a predetermined level, provides an output that changes with the amount of sensed charge to produce a signal at the amplifying transistor that is representative of the amount of charge collected. This signal is applied to an interface and passed to a sense amplifier so that when the signal passes a predetermined threshold, the counter is triggered and is incremented. Triggering the counter also commands a circuit element to clear or restore the predetermined bias level to the control gate (the sensing electrode) of the amplifying transistor. The control gate and interface are configured to have a very high impedance which creates some of the benefits of the invention. The counter circuitry provides a digital output representative of the physical phenomena.
In one embodiment, the voltage level (bias) of either the sensing or reference electrode can be switched relative to the other upon receipt of a triggering pulse. This changes the polarity of the electric field to cause charge of the opposite polarity to be driven to the sensing electrode, thereby eliminating the need to electrically connect a discharge path to the sensing electrode to clear the charge accumulated at the sensing electrode. This can be supplemented by capacitively coupling a compensation signal to the sensing electrode to cause the amplifier output signal to lessen in magnitude below a threshold level that permits additional charge or current measurements of the same polarity before performing bias reversal. These methods of clearing accumulated charge are a significant improvement over standard circuits which employ electrically noisy and expensive high value resistors to remove the charge accumulated on the sensing electrode, since elimination of this noise improves the performance of sensors which produce small currents by increasing their lower limits of detection, dynamic range and/or linearity.
Alternately or in combination with bias reversal and capacitive compensation, sensor performance can be improved by minimizing inaccuracies caused by leakage currents or current drawn from the sensor. The leakage currents are neutralized by the use of a controlled current source that induces current of a polarity opposite to the leakage current to flow to the sensing electrode. The controlled current source can be replaced or supplemented by a rate compensation circuit that compares the sensor output to the rate measured by an amplifier not connected to the sensing electrode, thereby providing an indication of the effect that the leakage current has on the sensor output to allow its correction. Other described methods of reducing leakage currents that can be used alone or in combination with the aforementioned features include the use of guard rings, physical switches or relays, the controlled creation of charges or currents of a specific polarity in a specific region of the sensing medium, controlled leakage over the surface of an insulator, and controlling the environment in which the circuit operates.
One preferred embodiment of the invention is described for use in an ionizing radiation detector which accurately measures both the total radiation exposure (with total charge reflecting the total dose) and the rate of exposure (with current reflecting the dose rate) using ionizable gas as the sensing medium. Alternate preferred embodiments of the invention permit use of the invention with sensing media other than ion chambers, including piezoelectric materials, photoelectric materials, liquid electrochemical materials, vacuum/gas/liquid/solid charge conduction materials, inductive pick-ups or coils, electric field measuring antennas, or even a surface with charge emission or work function changes. The electrical properties and/or size of many of these alternate sensing media do not require physically coupling the signal processing circuitry to the sensing device using an integrated circuit, thereby allowing use of discrete components to process the detected signal. However, where small size or low power is desired, and the number of devices to be produced is sufficient to justify the design and manufacturing costs, the circuit can be implemented on an integrated circuit. Approaches are described for manufacturing the invention to eliminate sources of leakage currents, including opening protective conductors after manufacture as needed.
The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, together with further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings.